Download Biogeographic Crossroads as Priority Areas for Biodiversity

Biogeographic Crossroads as Priority Areas for
Biodiversity Conservation
SACHA SPECTOR*
Department of Ecology and Evolutionary Biology, University of Connecticut, 75 N. Eagleville Road, U-43 Storrs,
Connecticut 06269, U.S.A.
Abstract: Threats to biodiversity outpace the resources of the conservation community and necessitate careful prioritization of conservation actions. I suggest that targeting the regions where biogeographic assemblages intersect—“biogeographic crossroads”—is a strategy that may achieve significant conservation economy by focusing on areas that satisfy many conservation criteria. I used a combination of data on Scarabaeine
beetles in Bolivia and on other taxa and locations from the literature to consider the short- and long-term
benefits of conserving these biogeographic crossroads. Biogeographic crossroads are areas of high species
richness and beta diversity, often across many taxonomic groups. They are also regions where representativeness can be achieved with relative efficiency. Recent evidence that ecotones may be loci of evolution suggests
that evolutionary processes such as speciation and coevolution may be conserved at biogeographic crossroads. Biogeographic crossroads appear to be areas of high conservation priority and opportunity in both the
short and long term and require increased attention in the process of setting conservation priorities.
Intersecciones Biogeográficas como Áreas Prioritarias para la Conservación de la Biodiversidad
Resumen: Las amenazas a la biodiversidad rebasan los recursos de la comunidad conservacionista y requieren de una cuidadosa priorización de las acciones de conservación. Sugiero que enfocar en las regiones
donde intersectan ensambles biogeográficos—“intersecciones biogeográficas”—es una estrategia que puede
lograr una economía significativa de los esfuerzos de conservación al atender áreas que satisfacen muchos
criterios de conservación. Utilicé una combinación de datos de escarabajos Scarabaeine de Bolivia y de otros
taxones y localidades de la literatura para considerar los beneficios a corto y largo plazo de conservar estas
intersecciones biogeográficas. Las intersecciones biogeográficas son áreas de alta riqueza de especies y de diversidad beta, y probablemente éste sea el caso de muchos grupos taxonómicos. También son regiones en las
que se puede alcanzar representatividad con relativa eficiencia. Evidencia reciente de que los ecotonos
pueden ser sitios de evolución sugiere que los procesos evolutivos tales como la especiación y coevolución
pueden ser conservados en intersecciones biogeográficas. Las intersecciones biogeográficas parecen ser áreas
de alta prioridad y oportunidad de conservación tanto a corto como a largo plazo y requieren mayor atención en el proceso de definición de prioridades de conservación.
Introduction
The magnitude of the threats to biodiversity, coupled
with the limited financial, technical, and physical re-
*Current address: Center for Biodiversity and Conservation, American Museum of Natural History, Central Park West at 79th Street,
New York, NY 10024, U.S.A., email [email protected]
Paper submitted December 26, 2000; revised manuscript accepted
December 3, 2001.
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Volume 16, No. 6, December 2002
sources available for biodiversity conservation, requires
careful prioritization of our efforts. Previous efforts to
formalize the process of setting conservation priorities
have been concentrated at two scales: global (or continental) and local. Global efforts have focused on documenting worldwide patterns of biological diversity to
identify biodiversity “hotspots” (Mittermeier et al. 1998;
Myers et al. 2000) or delineate ecoregions (Dinerstein et
al. 1995; Olson & Dinerstein 1998) with important biodiversity elements so that conservation efforts can be con-
Spector
centrated there. Despite some criticism ( Prendergast et
al. 1993), the strategy seems to be achieving positive results, at least terrestrially ( Balmford & Long 1995). At
more local scales, a variety of algorithms have been developed for computing the optimum area and configurations of reserve networks ( McKenzie et al. 1989; Davis
1996; Freitag et al. 1997; Rodrigues et al. 1999; Rothley
1999). The algorithms are used to find the minimum
area and most feasible spatial arrangement of reserves so
as to protect a specific proportion of the region’s species or populations (Pressey et al. 1993).
Recently, conservation practitioners have begun filling the “mesoscale” gap in our ability to identify areas of
high conservation priority. An increasing focus on conserving large, connected landscapes and regions of rapid
species turnover ( high beta diversity) is evident in conservation planning efforts ( Dinerstein et al. 1995; Wikramanayake et al. 1998; Foreman et al. 2000). I suggest
that targeting the regions where biogeographic assemblages intersect—“biogeographic crossroads”—is a strategy that complements this new mesoscale approach.
Biogeographic crossroads present conservationists
with opportunities to achieve significant economy of effort by focusing on relatively narrow areas that simultaneously satisfy many of the major criteria for establishing conservation priority (for more extensive treatments
of priority-setting criteria, see Margules & Pressey 2000;
also see Vane-Wright et al. 1991; Pressey et al. 1993; Mittermeier et al. 1998; Prendergast et al. 1999). Large biogeographic intersections create regions of rapid turnover (or high beta diversity) of species and habitats,
leading to exceptionally high levels of species richness
and creating the opportunity to meet the goals of representativeness and complementarity in protected-areas
systems. Protection of biogeographic crossroads may
also provide long-term biodiversity benefits by conserving evolutionary processes such as speciation and
coevolution.
I illustrate the benefits of conserving biogeographic
intersections with data from a series of surveys of a
widely proposed rapid biodiversity assessment group,
scarabaeine beetles (Coleoptera: Scarabaeidae: Scarabaeine) (Halffter & Favila 1993; Kremen et al. 1993;
Spector & Forsyth 1998) in South America. I emphasize
in particular one biogeographic crossroad region in eastern Bolivia that is currently protected within Parque Nacional Noel Kempff Mercado (PNNKM ) (Fig. 1). Of the
Huanchaca region where the 1.4 million-ha park is situated, Killeen (1998:83) states that “In no other part of
[South America] can one find five such dramatically different ecosystems in a single region.” Four years of work
in PNNKM by this author and others have yielded a
nearly complete picture of the scarabaeine fauna and
the habitat associations of each species (information on
collecting methods and sampling localities in PNNKM
are provided by Forsyth et al. [1998] and Spector & For-
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1481
syth [1998]). Thus, the park is an ideal area for examining biogeographic crossroads and their conservation
implications.
I have also included data on additional taxa at other
biogeographic crossroads that I encountered in the literature to emphasize that the high conservation value of
biogeographic crossroads is a broader phenomenon than
my own geographically and taxonomically restricted data
can demonstrate. A number of these additional biogeographic crossroads are the sites of globally important
protected areas. Although these parks were declared for
a variety of reasons, their common position at biogeographic crossroads highlights the potential for conserving similar regions elsewhere.
Potential Benefits of Biogeographic Crossroad Parks
Species Richness and Beta Diversity
Species richness is a commonly utilized criterion for assigning conservation priority ( Johnson 1995; Kerr 1997;
Mittermeier et al. 1998). The total richness of any given
area is a function of site richness (alpha diversity) and
the rate at which species replace one another between
sites (beta diversity) ( Whittaker 1972). Previous authors
have noted the role of biogeographic intersections in
producing high beta diversity and consequently species
richness (Fjeldså & Rahbek 1997), and recent approaches to designing conservation strategies have begun to emphasize zones of high beta diversity ( Wikramanayake et al. 1998).
The high levels of beta diversity at biogeographic
crossroads should be the characteristic most attractive
to conservationists. Naturally, the zones where two or
more biogeographic assemblages come into contact
have high beta diversity. And, by concentrating large
numbers of species in a single region, biogeographic
crossroads present us with the opportunity to protect
the species of two or more biogeographic zones within
a single protected area.
For example, the known scarabaeine fauna of PNNKM
currently holds at least 143 species. This is the highest
known scarabaeine richness of any Neotropical area of
its size—a full 12% of the approximately 1188 species
known from South America (Hanski & Cambefort 1991)
in 0.1% of the continent’s land area. One tribe in particular, the Phanaeini, a group of large, diurnal, colorful
species, exhibits extremely high richness in the park,
with 18 of the 74 species restricted to South America
( W. D. Edmonds, personal communication) occurring in
the park (24%).
Beta diversity is clearly the diversity component driving the overall high levels of richness in PNNKM. Any
given sampling site (a roughly 500-m transect) in the
park contained only a small subset of the park’s entire
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Volume 16, No. 6, December 2002
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Biogeographic Crossroads
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Figure 1. Partial map of North and
South America, showing the positions
of the biogeographic crossroad areas
discussed: 1, Parque Nacional Noel
Kempff Mercado (Bolivia); 2, Parque
Nacional Madidi (Bolivia); 3, Parque
Nacional Manu (Peru); 4, Sky Island/
Sierra Madre Region (United States
and Mexico).
fauna: on average, 29.4 (9.8) of the total 143 species
were found at a single site. Species in the tribe Phanaeini
showed a similar pattern, with only 4.8 (2.1) of the 18
species in the park occurring in any given site. Existing
data on the biogeography of the Phanaeini (Edmonds
1972, 1994) indicate that Phanaeine species in the park
are drawn from either the Amazonian Basin or the Cerrado Complex biogeographic assemblages; no species
occur in both regions. In fact, exactly half of PNNKM’s
18 phanaeine species are associated with each biogeographic assemblage. Thus, inclusion of both assemblages within PNNKM effectively doubles the number of
Phanaeine species under protection.
Located at another biogeographic crossroad, the Manu
Biosphere Reserve lies along the Andes-Amazon confluence in Peru. The reserve is almost the same size as
PNNKM (1.53 million ha) and protects the entire water-
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Volume 16, No. 6, December 2002
shed of the Rio Manu and parts of the Rio Alto Madre de
Dios watershed, encompassing a full complement of the
biological communities from the grassy Andean highlands at 4000 m to the lowland Amazonian forests at 240
m (Dallmeier et al. 1996). The Manu Biosphere Reserve
contains the highest known richness of carabid beetle
species in the world ( Erwin 1996).
Another Bolivian park, Parque Nacional Madidi, protects the entire elevational gradient from altiplano to
lowland forest. As in PNNKM, high levels of beta diversity produce high overall richness of scarabaeines (S.S.,
unpublished data). The lowland forests of the park contain a fairly unremarkable Amazonian community of approximately 45 species. If the series of habitats found
along the elevational gradient of the Andes is included,
however, the number of scarabaeine species nearly doubles. Similarly, scarab richness nearly doubles when the
Spector
Andean fauna is combined with the Amazonian lowland
fauna in Manu Biosphere Reserve (A. Forsyth, personal
communication).
Biogeographic Crossroads
1483
contains the greatest bee species richness in the world.
The greater Sky Island region is also known to have the
most diverse assemblages of ants, reptiles, and mammals
in North America ( Foreman et al. 2000).
Cross-Taxon Correspondence
One of the crucial questions faced in the prioritization
process is assessment of the degree to which a proposed
area will benefit multiple taxa, not just the focal or indicator taxon (or taxa) used to assign conservation priority. Prendergast (1997 ) and others ( Pearson & Cassola
1992; Oliver et al. 1998) have shown that, locally, the
patterns of richness and rarity across taxa often do not
covary. Although this may signify a limitation of indicator taxa at fine scales, at the larger scale of ecoregions
and biogeographic divisions, the correlation of various
taxa is likely to be high ( Reid 1998; Pearson & Carroll
1999). Major geological, climatic, and geometric features that cause important biogeographic patterns are
likely to act on most taxa in parallel, creating zones of
high turnover and richness. For example distributions of
species richness at the global scale are often repeated in
a variety of taxonomic groups, which makes the hotspot
approach effective (Myers et al. 2000). At somewhat
finer scales, regions with high topographic relief and
complex patterns of soils create an ecological heterogeneity to which most species are likely to respond.
This is certainly the case in PNNKM, where, in addition to scarab beetles, species richness is also high for
birds, reptiles, fishes, and mammals. The park protects
the richest assemblage of canids in South America (four
or possibly six species) (Bates et al. 1998; Emmons
1998; Harvey 1998; Sarmiento 1998), and there are currently more than 2700 species of vascular plants known
in the park (Killeen 1998).
In southern Peru, the Tambopata–Candamo Reserved
Zone protects an Andean-Amazonian gradient similar to
that of Alto Madidi National Park and Manu Biosphere
Reserve but has an added advantage of being situated at
the boundary of Holdridge’s (1967 ) tropical and subtropical life zones. Tambopata-Candamo is the most species-rich site in the world for spiders, odonates (dragonflies and damselflies), cicindelids (tiger beetles), asilids
(robber flies), and tabanids (horse flies) (Pogue 1996).
Another biogeographic crossroad, the Sky Island/Sierra
Madre region of Arizona, New Mexico, and northern
Mexico, is situated at the intersection of the subtropical
Sierra Madre assemblage and the temperate Mogollon
Highlands (Foreman et al. 2000). The Sky Islands are “the
northern limit of 14 plant families and four bird families
and the southern limit of seven bird families” ( Warshall
1994, as cited by Foreman et al. 2000). All of these colliding ranges combine to produce high beta diversity and
species richness across multiple taxa. Over half the bird
species in North America occur in the region (Felger &
Wilson 1994, as cited by Foreman et al. 2000), and it
Representativeness
Many efforts to set conservation priorities have incorporated the concept of representativeness into the ranking
process ( Johnson 1995). Representativeness seeks to include all the different species or ecosystem types of a
given area within a protected-area system (Kitching 1996;
Williams et al. 1996a, 1996b). Biogeographic crossroads
are regions where representativeness can be achieved
with relative economy, because beta-diverse biogeographic crossroads should permit greater numbers of species, communities, or ecosystem types to be conserved in
a concentrated area. Moreover, the greater the fidelity of
species to their respective biogeographic assemblages,
the greater the opportunity to conserve complementary,
nonoverlapping biotas within a single protected area.
In PNNKM scarabs, habitat fidelity is high. More than
80% of the park’s 143 scarab beetle species are entirely
restricted to either the Amazonian forests or the Cerrado
Complex shrublands and grasslands. Thus, the park is
conserving at least two distinct faunas and including
both major assemblages. Further, PNNKM is the only Bolivian park in which many of the cerrado specialist species are found, so the cerrado habitats of the park have a
high complementarity within the system of Bolivian protected areas.
Evolutionary Hotspots
Another strongly argued conservation priority is the
need to preserve evolutionary processes (Erwin 1991;
Vane-Wright et al. 1991; Brooks et al. 1992). It has been
difficult to translate this into direct conservation action,
perhaps because it is difficult to identify evolutionary
processes in space and time. Which evolutionary processes concern us? What sorts of landscapes harbor important evolutionary processes? What areas are “hotter”
than others evolutionarily?
Interestingly, biogeographic crossroad parks may hold
promise for conserving the dynamic processes of speciation, cladogenesis, and coevolution. Ecological heterogeneity, the essential feature of biogeographic crossroad
parks, is the root of the variety of selective regimes to
which species adapt. But classical evolutionary theory
maintains that gene flow between populations effectively
counters the effects of divergent selection on connected
populations (Mayr 1963). Recent evidence suggests, however, that ecological heterogeneity and strongly differing
selection pressures across ecotones may drive rapid evolutionary changes in populations even in the face of
gene flow, perhaps driving evolution and speciation at
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the edges of habitats and enriching biotas (Enserink
1997; Smith et al. 1997; Schneider et al. 1999). Thus, at
large scales the intersections of biogeographic zones
may be evolutionarily active zones—the origination sites
of new taxa or adaptations in existing taxa.
There is some evidence that speciation and divergence has occurred among the scarabs of PNNKM. Several congeneric pairs of ecological analogues exist
across the boundaries of the Amazon-Cerrado complex
in the park, suggesting that the close association of two
such disparate habitats might have played a role in the
diversification of Scarabaeines in the region. For example, Oxysternon silenus and O. conspicillatum, forest
species, are closely juxtaposed with O. palaemo in the savannas and cerrrados. Similarly, Phanaeus chalcomelas,
bispinus, and alvarengai are all forest-restricted species,
whereas their congeners P. kirbyi, palaeno, and melibaeus are all limited to the nearby the savannas and cerrados (S. S., unpublished data).
Higher taxon evolution at biogeographic crossroads
may have also played a role in generating the regional
richness. P. palaeno and O. palaemo share a unique
character, setae on the metasternum (Edmonds 1994).
The two species are certainly correctly placed in their
respective genera, but the shared character brings up
the question as to whether the more derived genus Oxysternon may have diverged from Phanaeus at the intersection of the Amazonian and Cerrado biotas.
Applying the Biogeographic Crossroad Strategy
Targeting biogeographic crossroads as conservation priorities should be seen as complementary to both largerand smaller-scale strategies for setting conservation priorities. One possible approach would be to first focus
on global hotspots and then on the important biogeographic crossroads within the hotspots. Similarly, existing
ecoregional analyses could be used to identify regions of
high complexity within an ecoregion ( Dinerstein et al.
1995; Ricketts et al. 1999). Where the spatial configuration
of ecoregions brings multiple ecoregions into contact,
biogeographic crossroad parks can be established to
straddle the transitions. PNNKM is such a region, and many
others easily can be found. Alternatively, when reserveoptimization algorithms are used at the local or regional
scale, zones of high beta diversity could be targeted;
contiguous grid squares across the intersection zones
could be given greater weight to ensure prioritization of
all sides of the intersection zone.
In some cases, biogeographic crossroad parks may be
aimed at conserving areas of outstanding richness, such
as the Madidi park envisioned by Remsen and Parker
(1995) that would have protected over 1200 bird species. In other situations, biogeographic crossroad parks
could be configured to overcome the difficulties of pro-
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Volume 16, No. 6, December 2002
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tecting species-poor areas of high conservation priority.
By pooling biotas in a single reserve, biogeographic
crossroad parks could raise the overall richness of the
proposed reserve to politically viable levels (for example, species-poor but critical areas around the confluence of the Argentine pampas, the Paraná flooded savannas, and the Uruguayan savannas).
Caveats
There are questions about this strategy that will bear further investigation. Foremost, perhaps, are the implications for species’ persistence in parks situated at the
edges of their ranges. Conventional wisdom holds that
the edges of species’ ranges tend to harbor more variable, more fragmented, and ultimately more extinctionprone populations (Brown 1984; Brown 1995). If this is
the case, then conserving many species at the edges of
their ranges may focus efforts on peripheral or perhaps
even sink populations whose persistence is uncertain.
Although the macroecological pattern of lower and
more variable population densities at range peripheries
may be valid, a series of recent studies suggests that
“conservation biologists . . . should reevaluate the assumption that populations inhabiting small, peripheral,
or otherwise isolated ecosystems are doomed to extinction” ( Lomolino & Channell 1995). Instead, Lomolino
and Channell (1995) and Channell and Lomolino (1998;
2000) have demonstrated that many endangered species
are persisting on the peripheries of their ranges, not in
the cores. Rather than precarious demographic sinks,
Lomolino and Channell assert, peripheral populations
are more numerous and pre-adapted to a wider variety
of ecological conditions, making it more likely that some
of those peripheral populations will be among the survivors of environmental change. Alternatively, peripheral
populations may simply be the last to face extinction
forces that spread like a contagion across the landscape.
Regardless of which explanation is correct, this new understanding of the importance of conserving species at
the peripheries of their ranges implies that biogeographic crossroads are not necessarily a recipe for conserving the “living dead.”
Importantly, in many of the world’s hotspots, remaining
habitat lies in scattered patches at the edges of historical
ranges (Brooks 2000). In these cases, the theoretical implications of abundance, population growth, and range position become secondary, and peripheral areas take on
higher priority as the last opportunities to conserve representatives of vanishing ecosystems. Biogeographic crossroad parks may fit well with this scenario by protecting the
edges of ranges where species and communities are persisting in the face of habitat loss in the cores of ranges.
Another question that will require consideration is
how large biogeographic crossroad parks must be to
function; the answer will likely be “it depends.” Ulti-
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mately, it will depend on the size of the ecological gradients occurring at the biogeographic crossroad and the
processes that maintain the habitats of each ecoregion.
In PNNKM, for example, maintenance of the current fire
regime within the park will be critical to sustaining savannas and other relatively open habitats in PNNKM,
and the park must be large enough to contain the dynamic movement of boundaries between forest and savanna which fire creates. The scale at which these key
processes occur will dictate the scale at which biogeographic crossroad parks must be designed.
Conclusion
Biogeographic intersections are dominant landscape features in all regions of the world. In South America, for
example, major intersection zones occur where the Amazon Basin meets savannas and shrublands on its southern and northern edges, semideciduous forests on its
southwestern edge, Guiana shield forests on its northeastern edge, and montane forests where the basin
meets the Andes. The Andes in turn meet Caribbean and
Pacific coastal forests, coastal deserts, pampas, and temperate rainforests as they run from Colombia to Tierra
del Fuego. In short, the opportunities for conserving
these regions exist in all corners of South America and,
presumably, the globe. Perhaps the commonness of biogeographic crossroads on the landscape makes their
identification and protection an easily understood and
politically workable strategy.
Biogeographic crossroads appear to provide conservation strategists with opportunities to simultaneously conserve high species richness, zones of high beta diversity
and complementarity, and evolutionary processes. Margules and Pressey (2000) state that reserves should be
judged on their success in achieving representativeness
and long-term persistence. If this is true, then increased
attention to the opportunities offered by conserving biogeographic crossroads will contribute to the process of
setting conservation priorities and designing reserves.
Acknowledgments
I thank A. Forsyth for early conversations about the ideas
in this paper; W. D. Edmonds for confirming determinations of Phanaeine taxa and for providing biogeographic
information for South American Phanaeines; B. D. Gill
and F. Genier for taxonomic determinations of much of
the collected material; S. Ayzama, F. Guerra, N. Araujo,
T. Guttierrez, and I. Garcia for their assistance in the
field; and R. K. Colwell, E. Sterling, R. Dunn, J. M. Morales,
and N. Stork for reading earlier versions of this manuscript
and providing many helpful suggestions for its improvement. I also thank E. Dinerstein, M. Lomolino, and an anon-
Biogeographic Crossroads
1485
ymous reviewer for helping to greatly clarify this manuscript and inject current experiences from the conservation
trenches. Portions of the survey work were supported
by the John D. and Catherine T. MacArthur Foundation
and the W. Alton Jones Foundation as part of a series of
field–based training workshops conducted by Conservation International. The American Museum of Natural History provided access to its research library collections.
The author was supported by a Graduate Research
Training Fellowship from the National Science Foundation and a Graduate Student Research Program Fellowship from the National Air and Space Administration.
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